A variety of alternative embodiments of multiple booms are already known. One of the most widespread solutions is used on several models of the applicant's own excavators. In this case, the lower end of the back boom is attached to a drag link on the frame of the revolving superstructure. The upper end of the upward-pointing back boom takes the form of an upward-opening fork, in which the rear area of the main boom is accepted by an articulated joint at a certain distance in front of its rear end, seen in the direction of movement of the excavator.
To facilitate the swivel movement of the back boom, a hydraulic boom cylinder is provided, one end of which acts on the articulated joint between the back boom and the main boom and the other end on a knuckle on the frame at a certain distance from the drag link of the back boom.
A hydraulic adjustment cylinder, lying to the rear and thus pulling under load, acts on an articulation pin located on the rear of the main boom to raise and lower it, the lower end of said cylinder being mounted on a bracket on a fixed pivot in the lower area of the back boom.
The front end of the main boom, seen in the direction of movement of the excavator, also takes the form of an open fork, to which the rear part of the stick boom is articulated at a certain distance from its rear end. Swivel movement of the stick boom is carried out by a hydraulic stick boom cylinder mounted above the main boom, the rear end of which is articulated to the main boom and the front end of which acts on the rear end of the stick boom.
A tool, usually a backhoe, is mounted on the front end of the stick boom. The tool is swiveled by a backhoe cylinder mounted above it on the stick boom, the rear end of which, analogous to the description above, is articulated to the stick boom and the front end of which is attached to the tool, either directly, or by a connecting rod and/or oscillating crank.
This solution has certain disadvantages, namely that the articulated joint between the back boom and the main boom must be located as high as possible in order to achieve maximum boom reach, so that the boom cylinder is required to be as long as possible. However, road traffic regulations place a limit on its length, because the height of vehicles on the public highway must not exceed 4 meters. This articulated joint is therefore located just below the upper edge of the main boom. However, this means that the upward-opening fork of the back boom must have relatively long sides in order to accept the entire cross-section, said sides having to be very strong, and thus cost-intensive, due to the high forces acting at this point. In addition, this fork must have an apex angle of around 180° to ensure that the main boom can move through an angle of around 90° in relation to the back boom, thereby placing a further restriction on the strength of the fork. For these reasons, there are limits on the ability of the fork to accept higher forces, despite a very strong embodiment.